The present invention relates to an optical pickup apparatus wherein a light flux emitted from a light source is converged by a light-converging optical system on an information recording surface through a transparent base board of an optical image recording medium, and whereby information is recorded on the information recording surface or information on the information recording surface is reproduced (recording/reproduction), and more particularly, to an optical pickup apparatus wherein a first optical information recording medium whose transparent base board has a thickness of t1 and a second optical information recording medium whose transparent base board has a thickness of t2 (provided that t2.noteq.t1) are used as an optical information recording medium. The invention further relates to a compensation element for the optical pickup apparatus stated above.
With a shortwave red semiconductor laser which has lately been put to practical use, there has been advanced development of a DVD (digital video disk) which is a high density optical information recording medium having a larger capacity in its size similar to that of a CD (compact disk) that is a conventional optical information recording medium (another name: optical disk). In the DVD, numerical aperture NA of an objective lens on the part of an optical disk is 0.6 when a shortwave semiconductor laser of 635 nm or 650 nm is used. Incidentally, compared with a CD, the DVD is of higher density with a track pitch of 0.74 .mu.n and a shortest pit length of 0.4 .mu.m which respectively are less than 1.6 .mu.m and 0.83 .mu..mu. both representing a track pitch and a shortest pit length for the CD. In addition to the CD and DVD mentioned above, optical disks of various standards, such as, for example, DVD-RAM (rewritable digital video disk), CD-R (write-once compact disk), LD (laser disk), MD (mini-disk) and MO (magnet-optic disk) are planned to be commercialized or are already widespread.
In the present age, various optical disks which are different each other in many points such as a size, a base board thickness, recording density and a wavelength to be used are on the market, and optical pickup apparatuses functionable with the various optical disks have been proposed. As one of them, there has been proposed an optical pickup apparatus wherein light-converging optical systems complying with different optical disks are provided and the light-converging optical systems are switched to be used depending on an optical disk to be reproduced. In this optical pickup apparatus, however, plural light-converging optical systems are required to cause high cost, a driving mechanism for switching the light-converging optical systems is required to cause an apparatus to be complicated, and accuracy for the switching is also required, which makes this optical pickup apparatus to be unpreferable.
Under the background stated above, there have been proposed various optical pickup apparatuses capable of recording information on and reading information from plural optical disks (hereinafter referred to as recording/reading) by using one light-converging optical system. As one of them, TOKKAIHEI 7-57271 discloses an optical pickup apparatus wherein an objective lens which is designed so that a wave front aberration owned by a beam to be converged is 0.07 .lambda. or less is used for the first optical disk with a transparent base board having a thickness of t1, while a converged spot is formed under the slight defocusing state for the second optical disk with a transparent base board having a thickness of t2.
However, in the optical pickup apparatus described in the above-mentioned publications, one objective lens is used for reproduction of two optical disks. Therefore, when reading one of the two optical disks, flare is caused, and when detecting a luminous flux reflected on an optical disk, a linear range of focus error signals is narrow, and stability of focusing control is lowered.